Talk by Yoshua Bengio from the University of Montreal. Given to the Redwood Center for Theoretical Neuroscience at UC Berkeley.
We explore the following crucial question: how could brains potentially perform the kind of powerful credit assignment that allows hidden layers of a very deep network to be trained and that has been so successful with deep learning recently? Global reinforcement learning signals have too much variance (scaling with the number of neurons or synapses) to be credible (by themselves) from a machine learning point of view. Concerns have been raised about how something like back-propagation could be implemented in brains. We present several intriguing results all aimed at answering this question and possibly providing pieces of this puzzle. We start with an update rule that yields updates similar to STDP but that is anchored in quantities such as pre-synaptic and post-synaptic firing rates and temporal rates of change. We then show that if neurons are connected symmetrically and thus define an energy function, (a) their behaviour corresponds to inference, i.e., going down the energy, and (b) after a prediction is made on a sensor and an actual value is observed, the early phases of inference in this network actually propagate prediction error gradients, and (c) using the above STDP-inspired rule yields a gradient descent step on prediction error for feedforward weights. This is based on a new mathematical result which provides a more general framework for machine learning to train dynamical systems at equilibrium. Finally, we discuss some of the limitations of the current model (such as the forced symmetry of synaptic weights and the question of learning the full joint distribution and not just a point prediction, and how to train dynamical systems which are generally not near their equilibrium points) as well as ideas around them.